In the production of yarns and similar filamentary materials, they are generally continuously checked with respect to the diameter produced or the already existing diameter or the thread thickness and modifications are made to the production process if there are found to be unacceptable variations from the required diameter. These variations can relate to small areas of the thread, i.e. short lengths, such as knot-like structures which occur when spinning textile threads, or over larger areas or lengths as occur in the production of metal filaments. In order to be able to detect these unallowable variations as early as possible and before long lengths of the faulty material have been produced, the monitoring apparatus must be able to recognize short, transient variations just as reliably as slow, trend-forming variations from the very time they start.
Thus, for example, an electronic yarn cleaner continuously monitors a thread running off a bobbin to establish whether the yarn diameter produced differs from the desired diameter and, in the event of an unacceptable variation, a cutting device is actuated to cut the thread. During the subsequent knotting process, the defective point can be removed from the yarn. In connection with this monitoring process, it is important to be able to measure the moving yarn in a contact-free manner and to obtain an electrical signal which is exactly proportional to the thread diameter.
Thus, in practice, a suitable measuring apparatus generally, but not necessarily has a light source and a photovoltaic cell arranged with a minimum spacing. A thread passing through the central area of this measuring arrangement reduces, as a function of its diameter, the quantity of light striking the photovoltaic cell and, in proportion thereto, the voltage of current delivered by the cell. This modification in the behavior of the photovoltaic cell caused by the thread diameter is amplified to the appropriate signal level by using known means and becomes the thread signal for further signal evaluation.
There is considerable interference to the thread signal when using a stable light source, e.g. a filament lamp, as a result of the ambient light striking the photovoltaic cell, but also due to the virtually unavoidable 100 or 120 Hz content of the electric room illumination. Therefore, it is preferred that only semiconductor devices (light-emitting diodes) are used as the light source because these can be operated with electric pulses of a relatively high frequency.
It is conventional practice to use pulse repetition frequencies of about 100 kHz to be able to detect short-length irregularities in the diameter when the thread is moving at high speed. Such a pulse repetition frequency, as the thread signal carrier, can be transmitted substantially free from the indicated environmental influences by a filter with high-pass characteristics in the subsequent amplifier.
As normal high-pass filters have a relatively large band width, interfering environmental influences with corresponding high frequency contents can still lead to interference with the thread signal. Such interference is due e.g. to light sources, mainly from the fluorescent tubes usually used for room illumination or other gas discharge lamps. As is known, in this type of illumination, the ionization is extinguished during each zero crossing of the AC voltage and ignited again at a predetermined voltage value of the following half-wave. At the time of reignition there is a very rapid rise of the light intensity, so that the differential content of the room illumination reaching the photoreceiver can superimpose pulses of 100 to 120 Hz on the thread signal, in spite of the use of a high-pass filter. Additional high frequency interference sources can be the inductive cross-talk on feed lines and transients resulting from the switching of electrical equipment.